The search for the presence of water on the Moon has attracted considerable attention and motivated several recent lunar missions, largely because of water's usefulness in making long-term lunar habitation feasible.
The Moon was believed to be generally anhydrous after analysis of Apollo mission soil samples. It was understood that any water vapor on the surface would generally be decomposed by sunlight, leaving hydrogen and oxygen lost to outer space. However, subsequent robotic probes found evidence of water, especially of water ice in some permanently shadowed craters on the Moon. In 2018, water ice was confirmed in multiple locations.Water and Ices on the Moon", science.nasa.gov, fetched 11 June 2024Water on the Moon: Direct evidence from Chandrayaan-1's Moon Impact Probe. Published on 2010/04/07. This water ice is not in the form of sheets of ice on the surface nor just under the surface, but there may be small (less than about ) chunks of ice mixed into the regolith, and some water is chemically bonded with minerals. Other experiments have detected water molecules in the negligible lunar atmosphere, and even some in low concentrations at the Moon's sunlit surface.
On the Moon, water molecules (H2O) and hydroxyl groups (-OH) are not present as free water but are chemically bonded within minerals as hydrates and hydroxides, existing in low concentrations across the lunar surface. Adsorbed water is estimated to be traceable at levels of 10 to 1000 ppm. The presence of water may be attributed to two primary sources: delivery over geological timescales via impacts and in situ production through interactions of solar wind hydrogen ions with oxygen-bearing minerals.Elston, D.P. (1968) "Character and Geologic Habitat of Potential Deposits of Water, Carbon and Rare Gases on the Moon", Geological Problems in Lunar and Planetary Research, Proceedings of AAS/IAP Symposium, AAS Science and Technology Series, Supplement to Advances in the Astronautical Sciences., p. 441 Confirmed hydroxyl-bearing materials include glasses, apatite or , and novograblenovite or .
History of observations 19th century and earlier In the 16th century, Leonardo da Vinci in his Codex Leicester attempted to explain the luminosity of the Moon by assuming that the Moon's surface is covered by water, reflecting the Sun's light. In his model, waves on the water's surface cause the light to be reflected in many directions, explaining why the Moon is not as bright as the Sun.
In 1834–1836, Wilhelm Beer and Johann Heinrich Mädler published their four-volume Mappa Selenographica and the book in 1837, which established the conclusion that the Moon has no bodies of water on the surface nor any appreciable atmosphere.
20th century The possibility of ice in the floors of polar lunar craters was first suggested in 1961 by Caltech researchers Kenneth Watson, Bruce C. Murray, and Harrison Brown.
Earth-based radar measurements were used to identify the areas that are in permanent shadow and hence have the potential to harbour lunar ice: Estimates of the total extent of shadowed areas poleward of 87.5 degrees latitude are for the north and south poles, respectively. Subsequent computer simulations encompassing additional terrain suggested that an area up to might be in permanent shadow.
Apollo program Trace amounts of water were found in lunar rock samples collected by Apollo astronauts. This was initially assumed to be a result of contamination, and the majority of the lunar surface was generally assumed to be completely dry."It's Official: Water Found on the Moon", Space.com, 23 September 2009 However, a 2008 study of lunar rock samples revealed evidence of water molecules trapped in volcanic glass beads.
The first direct evidence of water vapor near the Moon was obtained by the Apollo 14 ALSEP Suprathermal Ion Detector Experiment, SIDE, on March 7, 1971. A series of bursts of water vapor ions were observed by the instrument mass spectrometer at the lunar surface near the Apollo 14 landing site.
Luna 24
On 18 August 1976, the Soviet Luna 24 probe landed at Mare Crisium, took samples from depths of 118, 143, and 184 cm of the lunar regolith, and returned them to Earth. In February 1978 Soviet scientists M. Akhmanova, B. Dement'ev, and M. Markov of the Vernadsky Institute of Geochemistry and Analytical Chemistry published a paper claiming a detection of water fairly definitively. Their study showed that the samples returned to Earth by the 1976 Soviet probe Luna 24 contained about 0.1% water by mass, as seen in infrared absorption spectroscopy (at about wavelength), at a detection level about 10 times above the threshold, although Crotts points out that "The authors... were not willing to stake their reputations on an absolute statement that terrestrial contamination was completely avoided."Crotts, Arlin (October 12, 2009). "Water on the Moon", The Space Review. Retrieved 13 November 2023 This would represent the first direct measurement of water content on the surface of the moon, although that result has not been confirmed by other researchers.Spudis, Paul D. (June 1, 2012). "Who discovered water on the Moon?", Smithsonian Magazine. Retrieved 13 November 2023.
Clementine
A proposed evidence of water ice on the Moon came in 1994 from the United States military Clementine probe. In an investigation known as the 'bistatic radar experiment', Clementine used its transmitter to beam radio waves into the dark regions of the south pole of the Moon. Echoes of these waves were detected by the large dish antennas of the Deep Space Network on Earth. The magnitude and polarisation of these echoes was consistent with an icy rather than rocky surface, but the results were inconclusive,Clementine Probe and their significance has been questioned.
Lunar Prospector The Lunar Prospector probe, launched in 1998, employed a neutron spectrometer to measure the amount of hydrogen in the lunar regolith near the polar regions. It was able to determine hydrogen abundance and location to within 50 parts per million and detected enhanced hydrogen concentrations at the lunar north and south poles. These were interpreted as indicating significant amounts of water ice trapped in permanently shadowed craters,Lunar Prospector Science Results NASA but could also be due to the presence of the hydroxyl radical (•OH) chemically bound to minerals. Based on data from Clementine and Lunar Prospector, NASA scientists have estimated that, if surface water ice is present, the total quantity could be of the order of .Prospecting for Lunar Water , NASANeutron spectrometer results In July 1999, at the end of its mission, the Lunar Prospector probe was deliberately crashed into Shoemaker crater, near the Moon's south pole, in the hope that detectable quantities of water would be liberated. However, spectroscopic observations from ground-based telescopes did not reveal the spectral signature of water.No water ice detected from Lunar Prospector, NASA website
Cassini–Huygens More suspicions about the existence of water on the Moon were generated by inconclusive data produced by Cassini–Huygens mission, which passed the Moon in 1999.
21st century Deep Impact In 2005, observations of the Moon by the Deep Impact spacecraft produced inconclusive spectroscopic data suggestive of water on the Moon. In 2006, observations with the Arecibo planetary radar showed that some of the near-polar Clementine radar returns, previously claimed to be indicative of ice, might instead be associated with rocks ejected from young craters. If true, this would indicate that the neutron results from Lunar Prospector were primarily from hydrogen in forms other than ice, such as trapped hydrogen molecules or organics. Nevertheless, the interpretation of the Arecibo data do not exclude the possibility of water ice in permanently shadowed craters. In June 2009, NASA's Deep Impact spacecraft, now redesignated EPOXI, made further confirmatory bound hydrogen measurements during another lunar flyby.
Kaguya As part of its lunar mapping programme, Japan's Kaguya probe, launched in September 2007 for a 19-month mission, carried out gamma ray spectrometry observations from orbit that can measure the abundances of various elements on the Moon's surface.Kaguya Gamma Ray Spectrometer, JAXA Japan's Kaguya probe's high resolution imaging sensors failed to detect any signs of water ice in permanently shaded craters around the south pole of the Moon, and it ended its mission by crashing into the lunar surface in order to study the ejecta plume content.
Chang'e 1 The People's Republic of China's Chang'e 1 orbiter, launched in October 2007, took the first detailed photographs of some polar areas where ice water is likely to be found."Who's Orbiting the Moon?" , NASA, February 20, 2008
Chandrayaan-1
India's ISRO spacecraft Chandrayaan-1 released the Moon Impact Probe (MIP) that impacted Shackleton Crater, of the lunar south pole, at 20:31 on 14 November 2008 releasing subsurface debris that was analysed for presence of water ice. During its 25-minute descent, the impact probe's Chandra's Altitudinal Composition Explorer (CHACE) recorded evidence of water in 650 mass spectra gathered in the thin atmosphere above the Moon's surface and hydroxyl absorption lines in reflected sunlight.
On September 25, 2009, NASA declared that data sent from its M3 confirmed the existence of hydrogen over large areas of the Moon's surface, albeit in low concentrations and in the form of hydroxyl groups (OH) chemically bound to soil."Spacecraft see 'damp' Moon soils", BBC, 24 September 2009 This supports earlier evidence from spectrometers aboard the Deep Impact and Cassini probes."Moon crash will create six-mile plume of dust as Nasa searches for water", The Times, October 3, 2009Discovery of water on Moon boosts prospects for permanent lunar base, The Guardian, 24 September 2009 On the Moon, the feature is seen as a widely distributed absorption that appears strongest at cooler high latitudes and at several fresh feldspathic craters. The general lack of correlation of this feature in sunlit M3 data with neutron spectrometer H abundance data suggests that the formation and retention of OH and H2O is an ongoing surficial process. OH/H2O production processes may feed polar cold traps and make the lunar regolith a candidate source of volatiles for human exploration.
Although M3 results are consistent with recent findings of other NASA instruments onboard Chandrayaan-1, the discovered water molecules in the Moon's polar regions is not consistent with the presence of thick deposits of nearly pure water ice within a few meters of the lunar surface, but it does not rule out the presence of small (<~), discrete pieces of ice mixed in with the regolith. Additional analysis with M3 published in 2018 had provided more direct evidence of water ice near the surface within 20° latitude of both poles. In addition to observing reflected light from the surface, scientists used M3's near-infrared absorption capabilities in the permanently shadowed areas of the polar regions to find absorption spectra consistent with ice. At the north pole region, the water ice is scattered in patches, while it is more concentrated in a single body around the south pole. Because these polar regions do not experience the high temperatures (greater than 373 Kelvin), it was postulated that the poles act as cold traps where vaporized water is collected on the Moon.